The invention relates to a process for the nitration of toluene with nitrating acid to form dinitrotoluene (DNT) in a two-stage process, wherein the first stage is performed adiabatically and the second stage isothermally.
Dinitrotoluene (DNT) is the precursor of toluene diisocyanate, which is used as a raw material for the production of polyurethanes. The conventional industrial process for the production of dinitrotoluene is the isothermal two-stage reaction of toluene with nitrating acid, a mixture of nitric acid and sulfuric acid Ullmanns Encyklopädie der technischen Chemie, 4th edition, volume 17, page 392, Verlag Chemie, Weinheim 1979. In this process, a mixture of isomers of mononitrotoluene (MNT) is first produced, and is converted to dinitrotoluenes in a second, separate process step. However, the process has the disadvantage that the acid phase forming in both steps (substantially sulfuric acid) has to be freed of the water taken up, with high energy costs.
It is also known to convert toluene to dinitrotoluene in one step by an adiabatic route (EP-A-597 361). In this process, toluene is reacted with at least 2 equivalents of a nitrating acid of a specific composition by an adiabatic route, a final temperature of more than 120° C. being reached. After separating the phases at this temperature, the acid phase is fed into a concentrating stage (flash evaporation in vacuo), the heat content of the acid being utilised for the concentration. The concentrated acid is topped up with nitric acid and recycled into the process.
In this process, however, the difficulty arises that, during the flash evaporation, a certain portion of DNT dissolved in the acid passes into the gas phase together with the water distilling off and then solidifies during the condensation of the vapours under the condensation conditions of the water (melting point of the mixture of isomers 52–58° C.) and coats the heat exchanger. In addition, in the course of the process, temperatures are reached at which the DNT is not permanently stable in the presence of by-products. To guarantee the safety of the process, the permitted residence time of substance streams containing a high proportion of DNT at high temperatures must not be exceeded. This requires considerable expenditure on safety devices.
Various solutions have been proposed to the problem of interference with condensation caused by DNT contained in the vapours, such as alternately operated, staggered heat exchangers, contact condensers or injection condensers (R. A. Vauck, H. A. Müller, Grundoperationen chemischer Verfahrenstechnik, 5th edition, VEB Leipzig 1962, p. 447). In addition, EP-A-0696569 describes a process for the adiabatic production of DNT, wherein at least 5% MNT is deliberately retained in the reaction product of the adiabatic nitration to prevent the coating of the heat exchangers.
All these processes are costly from either a technical or an energy point of view, or they do not allow DNT to be produced with a very small proportion of MNT. Furthermore, the problem remains of the high temperature stress to which DNT or media containing high proportions of DNT are subjected.
The object of the present invention was therefore to provide a technically simple process for the production of dinitrotoluene by the nitration of toluene, in which at least part of the heat of reaction formed can be utilised for concentrating the waste acid and, at the same time, DNT and media containing DNT are not subjected to temperatures that are questionable from a safety point of view.